Acoustic sensor for downhole measurement tool

ABSTRACT

An acoustic sensor for use in a downhole measurement tool is provided. The acoustic sensor includes a piezo-composite transducer element. In various exemplary embodiments, the acoustic sensor further includes a composite backing layer, at least one matching layer, and a barrier layer deployed at an outermost surface of the sensor. Exemplary embodiments of this invention may advantageously withstand the extreme temperatures, pressures, and mechanical shocks frequent in downhole environments and thus may exhibit improved reliability. Exemplary embodiment of this invention may further provide improved signal to noise characteristics. Methods for fabricating acoustic sensors and downhole measurement tools are also provided.

FIELD OF THE INVENTION

The present invention relates generally to downhole measurement toolsutilized for measuring properties of a subterranean borehole duringdrilling operations. More particularly, this invention relates to anacoustic sensor for use in a downhole measurement tool, the acousticsensor having one or more piezo-composite transducer elements fortransmitting and/or receiving ultrasonic energy during measurement ofborehole characteristics.

BACKGROUND OF THE INVENTION

The use of acoustic (e.g., ultrasonic) measurement systems in prior artdownhole applications, such as logging while drilling (LWD), measurementwhile drilling (MWD), and wireline logging applications is well known.In known systems an acoustic sensor, typically with a substantiallyhomogenous piezo-ceramic structure on board, operates in a pulse-echomode in which it is utilized to both send and receive a pressure pulsein the drilling fluid (also referred to herein as drilling mud). In use,an electrical drive voltage (e.g., a square wave pulse) is applied tothe transducer, which vibrates the surface thereof and launches apressure pulse into the drilling fluid. A portion of the ultrasonicenergy is typically reflected at the drilling fluid/borehole wallinterface back to the transducer, which induces an electrical responsetherein. Various characteristics of the borehole, such as boreholediameter and measurement eccentricity and drilling fluid properties, maybe inferred utilizing such ultrasonic measurements. For example, U.S.Pat. No. 4,665,511 to Rodney et al., discloses a System for AcousticCaliper Measurements using ultrasonic measurements in a borehole, whileU.S. Pat. No. 4,571,693 to Birchak et al., discloses an Acoustic Devicefor Measuring Fluid Properties that is said to be useful in downholedrilling applications. Numerous other prior art acoustic measurementsystems are available in the prior art, including for example, U.S. Pat.No. RE34,975 to Orban et al., U.S. Pat. No. 5,469,736 to Moake, U.S.Pat. No. 5,486,695 to Schultz et al., and U.S. Pat. No. 6,213,250 toWisniewski et al.

While prior art acoustic sensors have been used in various downholeapplications (as described in the previously cited U.S. Patents), theiruse, particularly in logging while drilling (LWD) and measurement whiledrilling (MWD) applications, tends to be limited by various factors. Asused in the art, there is not always a clear distinction between theterms LWD and MWD, however, MWD typically refers to measurements takenfor the purpose of drilling the well (e.g., navigation) whereas LWDtypically refers to measurement taken for the purpose of analysis of theformation and surrounding borehole conditions. Nevertheless, these termsare hereafter used synonymously and interchangeably.

Most prior art acoustic measurement systems encounter serious problemsthat result directly from the exceptional demands of the drillingenvironment. Acoustic sensors used downhole must typically withstandtemperatures ranging up to about 200 degrees C. and pressures ranging upto about 25,000 psi. In many prior art systems, expansion andcontraction caused by changing temperatures is known, for example, tocause delamination of impedance matching layers and/or backing layersfrom surfaces of the transducer element. Further, the acoustic sensorsare subject to various (often severe) mechanical forces, includingshocks and vibrations up to 650 G per millisecond. Mechanical abrasionfrom cuttings in the drilling fluid, and direct hits on the sensor face(e.g., from drill string collisions with the borehole wall) have beenknown to damage or even fracture a piezo-ceramic element in thetransducer. A desirable acoustic sensor must not only survive the aboveconditions but also function in a substantially stable manner for up toseveral days (time of a typical drilling operation) while exposedthereto.

Existing acoustic measurement systems also tend to be limited indownhole environments by transducer ringing and a relatively poor signalto noise ratio (as compared to, for example, transducers used in otherapplications). As such, typical prior art acoustic sensors are typicallyimprecise at measuring distances outside of a relatively narrowmeasurement range. At relatively small distances (e.g., less than aboutone centimetre) acoustic measurements tend to be limited by residualtransducer ringing and other near field limitations related to thegeometry of the transducer. At relatively larger distances (e.g.,greater than about 8 centimetres) acoustic measurements tend to belimited by a reduced signal to noise ratio, for example, related to thetransmitted signal amplitude and the receiver sensitivity required toovercome drilling mud attenuation and formation/mud impedance contrasteffects.

Therefore, there exists a need for an improved acoustic sensor fordownhole applications. While the above described limitations are oftenassociated with the transducer element (i.e., the piezo-ceramic elementin prior art downhole devices), and thus represent a need for improvedtransducers for down hole applications, there also exists a need forimproved impedance matching layers and backing layers (also referred toas attenuating layers) for acoustic sensors utilized in downholeapplications. Thus a need especially exists for an acoustic sensorhaving an improved transducer element, impedance matching layers, andbacking layer specifically to address the challenging demands ofdownhole applications.

SUMMARY OF THE INVENTION

The present invention addresses one or more of the above-describeddrawbacks of prior art acoustic sensors used in downhole applications.Referring briefly to the accompanying figures, aspects of this inventioninclude a downhole tool including at least one acoustic sensor having apiezo-composite transducer. The piezo-composite transducer may beconfigured, for example, to withstand demanding downhole environmentalconditions. Various exemplary embodiments of the acoustic sensor furtherinclude a matching layer assembly for substantially matching theacoustic impedance of the piezo-composite transducer with that of thedrilling fluid and for providing mechanical protection for thetransducer and/or a backing layer for substantially attenuatingultrasonic energy reflected back into the acoustic sensor. Exemplaryembodiments of the downhole tool of this invention include threeacoustic sensors disposed substantially equidistantly around theperiphery of the tool.

Exemplary embodiments of the present invention advantageously provideseveral technical advantages. Various embodiments of the acoustic sensorof this invention may withstand the extreme temperatures, pressures, andmechanical shocks frequent in downhole environments. Tools embodyingthis invention may thus display improved reliability as a result of theimproved robustness to the downhole environment. Exemplary embodimentsof this invention may further advantageously improve the signal to noiseratio of downhole acoustic measurements and thereby improve thesensitivity and utility of such measurements.

In one aspect the present invention includes a downhole measurementtool. The downhole measurement tool includes a substantially cylindricaltool body having a cylindrical axis. The tool further includes at leastone acoustic sensor deployed on the tool body, the acoustic sensorincluding a piezo-composite transducer element with anterior andposterior faces. The piezo-composite transducer is in electricalcommunication with an electronic control module via conductiveelectrodes disposed on each of the faces. The piezo-composite transducerelement includes regions of piezoelectric material deployed in a matrixof a substantially non piezoelectric material, the regions extendingthrough a thickness of the transducer element in at least one dimension.In exemplary variations of this aspect, the acoustic sensor includes alaminate having a composite backing layer, at least one matching layer,and a barrier layer deployed at an outermost surface of the acousticsensor.

In another aspect, this invention includes an acoustic sensor having apiezo-composite transducer element. Further aspects of this inventioninclude a method for fabricating a downhole measurement tool and amethod for fabricating an acoustic sensor.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiment disclosed may be readily utilized as a basisfor modifying or designing other structures for carrying out the samepurposes of the present invention. It should be also be realized bythose skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of an offshore oil and/or gasdrilling platform utilizing an exemplary embodiment of the presentinvention.

FIG. 2 is a schematic representation of an exemplary MWD tool includingan exemplary embodiment of the present invention.

FIG. 3 is a cross sectional view as shown on section 3-3 of FIG. 2.

FIG. 4 is a schematic representation, cross sectional perspective view,of one embodiment of a piezo-composite transducer according to theprinciples of this invention.

FIG. 5 is a schematic representation, cross sectional perspective view,of another embodiment of a piezo-composite transducer according to theprinciples of this invention.

FIG. 6 is a schematic representation, cross sectional perspective view,of still another embodiment of a piezo-composite transducer according tothe principles of this invention.

FIG. 7 is a cross sectional schematic representation of the acousticsensor assembly 120 shown in FIG. 3.

FIG. 8A is a schematic representation, cross sectional perspective view,of one embodiment of the impedance matching layers discussed withrespect to FIG. 7.

FIG. 8B is schematic representation, cross sectional perspective view,of another embodiment of the impedance matching layers discussed withrespect to FIGURE 7.

FIG. 9A is a schematic representation, cross sectional perspective view,of one embodiment of the barrier layer discussed with respect to FIG. 7.

FIG. 9B is a schematic representation, cross sectional perspective view,of another embodiment of the barrier layer discussed with respect toFIG. 7.

FIG. 10 is a cross sectional schematic representation of an alternativeembodiment of an acoustic sensor assembly according to this invention.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates one exemplary embodiment of ameasurement tool 100 according to this invention in use in an offshoreoil or gas drilling assembly, generally denoted 10. In FIG. 1, asemisubmersible drilling platform 12 is positioned over an oil or gasformation (not shown) disposed below the sea floor 16. A subsea conduit18 extends from deck 20 of platform 12 to a wellhead installation 22.The platform may include a derrick 26 and a hoisting apparatus 28 forraising and lowering the drill string 30, which, as shown, extends intoborehole 40 and includes a drill bit 32 and an acoustic measurement tool100 including at least one acoustic sensor 120. Drill string 30 mayfurther include a downhole drill motor, a mud pulse telemetry system,and one or more other sensors, such as a nuclear logging instrument, forsensing downhole characteristics of the borehole and the surroundingformation.

It will be understood by those of ordinary skill in the art that themeasurement tool 100 of the present invention is not limited to use witha semisubmersible platform 12 as illustrated in FIG. 1. Measurement tool100 is equally well suited for use with any kind of subterraneandrilling operation, either offshore or onshore.

Referring now to FIG. 2, one exemplary embodiment of an acousticmeasurement tool 100 according to the present invention is illustratedin perspective view. In FIG. 2, measurement tool 100 is typically asubstantially cylindrical tool, being largely symmetrical aboutcylindrical axis 70 (also referred to herein as a longitudinal axis).Acoustic measurement tool 100 includes a substantially cylindrical toolcollar 110 configured for coupling to a drill string (e.g., drill string30 in FIG. 1) and therefore typically, but not necessarily, includesthreaded end portions 72 and 74 for coupling to the drill string.Through pipe 105 provides a conduit for the flow of drilling fluiddownhole, for example, to a drill bit assembly (e.g., drill bit 32 inFIG. 1). Measurement tool 100 includes at least one, and preferablythree or more, acoustic sensors 120 having a piezo-composite transducerelement (not shown in FIG. 2) configured for transmitting and receivingultrasonic signals. The piezo-composite transducer elements aredescribed in more detail below with respect to FIGS. 4 through 6.

Referring now to FIG. 3, the exemplary acoustic measurement tool 100 isshown in cross section as illustrated on FIG. 2. As shown on FIG. 3,downhole measurement tool 100 includes three acoustic sensors 120, eachof which is disposed in a housing 122. As noted above, however, theinvention is not limited to any particular number of acoustic sensorsthat may be deployed at one time. As described in more detail below, atleast one of the acoustic sensors 120 includes a piezo-compositetransducer element 140. Acoustic sensors 120 may optionally furtherinclude a matching layer assembly 150 for substantially matching theimpedance of the piezo-composite transducer 140 with drilling fluid atthe exterior of the tool 100 and/or for substantially shielding thepiezo-composite transducer element 140 from mechanical damage. Theacoustic sensors 120 may optionally further include a backing layer 160for substantially attenuating acoustic energy reflected back into thetool 100. Exemplary matching layer assemblies and backing layers aredescribed in more detail below with respect to FIGS. 7 through 10.

With continued reference to FIG. 3, the housings 122 are typicallyfabricated from metallic materials, such as conventional stainlesssteels, and typically each include one or more sealing members 112,e.g., o-ring seals, for substantially preventing the flow of drillingfluid from the borehole through to the interior 102 of the downholemeasurement tool 100. Suitable sealing assemblies include loaded lipseals such as a Polypack® seal, which are available from Gulf Coast Seal& Engineering Corporation (a distributor of Parker Seals), 9119 MonroeRd, Houston, Tex. 77061. The interface between the housing 122 and thesensors 120 may also include, for example, a molded Viton® bond seal 114(also available from Gulf Coast Seal & Engineering) for substantiallypreventing drilling fluid from penetrating into the interior of thehousing 122.

With further reference to FIG. 3, the acoustic sensors 120 are coupledvia connectors 124, for example, to a controller, which is illustratedschematically at 130. Controller 130 typically includes conventionalelectrical drive voltage electronics (e.g., a high voltage, highfrequency power supply) for applying a waveform (e.g., a square wavevoltage pulse) to the piezo-composite transducer 140, which causes thetransducer to vibrate and thus launch a pressure pulse into the drillingfluid. Controller 130 typically also includes receiving electronics,such as a variable gain amplifier for amplifying the relatively weakreturn signal (as compared to the transmitted signal). The receivingelectronics may also include various filters (e.g., low and/or high passfilters), rectifiers, multiplexers, and other circuit components forprocessing the return signal.

With still further reference to FIG. 3, a suitable controller 130 mightfurther include a programmable processor (not shown), such as amicroprocessor or a microcontroller, and may also includeprocessor-readable or computer-readable program code embodying logic,including instructions for controlling the function of the acousticsensors 120. A suitable controller 130 may also optionally include othercontrollable components, such as sensors, data storage devices, powersupplies, timers, and the like. The controller 130 may also be disposedto be in electronic communication with various sensors and/or probes formonitoring physical parameters of the borehole, such as a gamma raysensor, a depth detection sensor, or an accelerometer, gyro ormagnetometer to detect azimuth and inclination. Controller 130 may alsooptionally communicate with other instruments in the drill string, suchas telemetry systems that communicate with the surface. Controller 130may further optionally include volatile or non-volatile memory or a datastorage device. The artisan of ordinary skill will readily recognizethat while controller 130 is shown disposed in collar 110, it mayalternatively be disposed elsewhere within the measurement tool 100.

As stated above, and with yet further reference to FIG. 3, measurementtool 100 includes at least one acoustic sensor 120 having apiezo-composite transducer element 140. A composite material isgenerally defined as a synthetically produced material including two ormore dissimilar components to achieve a property or properties that arein at least one sense superior to that of any of the constituentcomponents. Known piezo-composite materials are typically fabricated bycombining, for example, a piezo-ceramic and a relatively soft (ascompared to the piezo-ceramic) non piezoelectric material (e.g., apolymeric material) to achieve a composite material having, for example,superior electromechanical properties. Embodiments of an acoustic sensorof this invention may utilize substantially any piezo-compositetransducer element fabricated from substantially any constituents, oneof which is a piezoelectric material. For example, the piezo-compositetransducer may include a 1-3 piezoelectric-polymer composite including aperiodic array of finely spaced piezoelectric posts extending throughthe thickness of the transducer, with each post surrounded on the sidesby a polymer matrix. Alternatively, the piezo-composite transducer mayinclude a 2-2 piezoelectric-polymer composite including alternatingtwo-dimensional strips of piezo-ceramic and polymer disposed side byside or a 0-3 piezoelectric-polymer composite including a piezoelectricpowder embedded in a polymer matrix.

Referring now to FIGS. 4 through 9, exemplary acoustic sensors suitablefor use in downhole measurement tools (e.g., measurement tool 100 ofFIGS. 1 through 3) according to the present invention are illustrated.FIG. 4 shows an exemplary piezo-composite transducer 240 having acomposite structure similar to a conventional 1-3 piezo-composite.Piezo-composite transducer 240 is substantially in the form of a diskand includes an array of piezoelectric posts 234 disposed in a nonpiezoelectric matrix 236. Piezoelectric posts 234 typically extendthrough the thickness of the transducer 240 in at lest one dimension andmay be disposed in substantially any predetermined pattern. While thepiezoelectric posts may be disposed in substantially any pattern, aconventional 1-3 pattern including alternating piezoelectric 234 and nonpiezoelectric 236 posts is often desirable owing to its relative ease ofmanufacturing (as compared with other, more complex patterns). Thepiezoelectric posts 234 may have substantially any lateral spacing 239,with finer spacing required for high frequency applications. For mostdownhole applications a lateral spacing 239 on the order of from about afraction of to several times the diameter (for cylindrical) orcross-sectional width (for square/rectangular) of the piezoelectricposts is suitable.

Referring now to FIG. 5, an alternative piezo-composite transducer 340is shown, having a composite structure similar to a conventional 2-2piezo-composite. Piezo-composite transducer 340 is substantially in theform of a disk optionally including two or more axial slits 325 disposedaround the periphery thereof. Transducer 340 preferably includes fouraxial slits 325 disposed at about ninety-degree intervals. The slits 325are believed to reduce lateral vibration modes and thus may be desirablefor certain piezo-composites (such as 2-2 family composites) and certaindownhole applications. While substantially any 2-2 piezo-compositestructure may be utilized for exemplary alternating planar layers ofpiezoelectric and polymer materials, transducer 340 includes apiezoelectric disk 342 about which a plurality of alternatingpiezoelectric rings 344A, 344B, 344C, and 344D and non piezoelectricrings 346A, 346B, 346C, and 346D are disposed. It will be understoodthat a general reference herein to the piezoelectric rings 344 and nonpiezoelectric rings 346 applies collectively to the piezoelectric rings344A, 344B, 344C, and 344D or non piezoelectric rings 346A, 346B, 346C,and 346D, respectively, unless otherwise stated. Transducer 340 mayinclude substantially any number of concentric piezoelectric rings 344.Typically, the greater the number of concentric rings the better theperformance of the piezo-composite (especially at relatively higherfrequencies), but with the trade-off of increased fabrication costs.Good performance at a reasonable cost may typically be achieved with twoor more piezoelectric rings 344.

In the embodiments shown on FIG. 5, the radial thickness of thepiezoelectric rings 344 decreases from the inner ring 344A to the outerring 344D according to a predetermined mathematical function (e.g.,according to a mathematical relation based on standard Gaussian orBessel functions). Likewise the thickness of the non piezoelectric rings346 increases from the inner ring 346A to the outer ring 346B. Suchvarying of the thicknesses of the piezoelectric 344 and/or the nonpiezoelectric 346 rings is referred to herein as apodization. Suchapodization, while not necessary, may be advantageous in that it tendsto reduce unwanted sidelobes and non transverse modes of vibration(i.e., vibration modes perpendicular to the cylindrical axis 370 of thetransducer 340), thereby increasing the magnitude of the usable acousticoutput for a given electrical input.

With continued reference to FIGS. 4 and 5, embodiments of thepiezo-composite transducer of this invention may be fabricated fromsubstantially any piezoelectric and non piezoelectric materials that arestable under downhole conditions (e.g., up to about 200 degrees C. andabout 25,000 psi). Piezoelectric materials selected from the leadzirconate titanates (PZT) or the lead metaniobates are typicallysuitable for many downhole applications. For some applications, it maybe desirable to utilize piezoelectric materials having a Curietemperature greater than about 250 degrees C. to prevent thepiezoelectric material from becoming either partially or fully deployedand thus altering the piezoelectric properties thereof under extremedownhole conditions (e.g., high temperature). Desirable piezoelectricmaterials also may typically be characterized as having anelectromechanical coupling coefficient (k) equal to or greater thanabout 0.3. Exemplary lead zirconate titanates useful in this inventioninclude PZT5A available from Morgan Electro Ceramics, Inc., 232 ForbesRoad, Bedford, Ohio, and K350 available from Keramos AdvancedPiezoelectrics, 5460 West 84^(th) Street, Indianapolis, Ind. ExemplaryLead Metaniobates useful in this invention include K81 and K85 availablefrom Keramos Advanced Piezoelectrics and BM940 available from SensorTechnology Limited, P.O. Box 97, Collingwood, Ontario, Canada.

Useful non piezoelectric materials typically include polymeric materialsthat are resistant to temperatures in excess of 200 degrees C., exhibitlow shrinkage on curing, and may be characterized as having a thermalcoefficient of expansion (CTE) less than about 100 parts per million(ppm) per degree C. Various useful non piezoelectric materials may alsobe characterized as having a glass transition temperature above about250 degrees C. Suitable non piezoelectric materials are furthergenerally resistant to thermal and mechanical shocks and mechanicallyflexible (i.e., low elastic modulus) and tough (i.e., high fracturetoughness) enough to accommodate thermal expansion and stress mismatchesbetween the various layers of the acoustic sensor. Desirable nonpiezoelectric materials are typically selected from conventional epoxyresin materials such as Insulcast® 125 epoxy resin available fromInsulcast®, 565 Eagle Rock Avenue, Roseland, N.J.

With further reference to FIGS. 4 and 5, piezo-composite transducersuseful in embodiments of this invention may be fabricated bysubstantially any suitable techniques. For example, transducer 240 (FIG.4) may be fabricated using a process similar to the known dice and filltechnique such as disclosed by Smith, Wallace A., SPIE, Vol. 1733, page10. Using such a process, two sets of substantially orthogonal groovesare cut (e.g., using a diamond saw) in a conventional piezo-ceramicblock (e.g., a piezo-ceramic disk). A non piezoelectric (e.g.,polymeric) material may then be cast into the grooves. The solidpiezo-ceramic base (having a thickness typically ranging from about 0.5to about 2 millimetres) is then ground (or cut) off and the compositepolished to a final thickness (e.g., from about 1 to about 2millimetres). Electrical communication may be established bysubstantially any known technique, for example, by sputter depositing athin layer of gold 280 (shown on FIGS. 4 and 5), for example, on each ofthe opposing faces of the piezo-composite disk and attachingconventional leads (not shown) thereto.

In an alternative fabrication procedure a piezo-ceramic slurry may becast (e.g., via conventional injection molding techniques) in a reversemold. After removal of the piezo-ceramic from the mold, a polymericmaterial may be cast into the open spaces therein to form thepiezo-composite. Any solid piezo-ceramic base may be ground or cut offand the piezo-composite polished to a final thickness as describedabove. Electrical leads may also be attached as described in thepreceding paragraph. Such a fabrication procedure, while typically moreexpensive than the dice and fill technique described above, mayadvantageously provide increased flexibility in fabricating more complexpiezo-composite structures, such as, for example, piezo-compositetransducer 340 shown in FIG. 5.

The artisan of ordinary skill will readily recognize that the abovedescribed piezo-composite transducers (shown in FIGS. 4 and 5) aremerely exemplary. A wide range of configurations and piezoelectric andnon piezoelectric materials may be suitable for downhole applications,depending upon device requirements, cost restraints, the particulardownhole conditions, and/or other factors. For example, as describedabove, acoustic sensors of this invention may utilize substantially any1-3 or 2-2 type piezo-composites. Additionally, it will be appreciatedthat embodiments of the piezo-composite transducers of this inventionmay include other materials (e.g., additional non piezoelectricmaterials and/or two or more distinct piezoelectric materials).

Piezo-composite transducers 240 and 340, as shown in FIG. 4 and 5, aretypically configured for conventional pulse echo ultrasonicmeasurements. However, piezo-composite transducers, in general, may alsoadvantageously provide for alternative ultrasonic measurement schemes,such as a pitch-catch scheme, in which one portion of thepiezo-composite transducer is utilized as a transmitter (i.e., totransmit an ultrasonic signal) and another portion of the transducer isutilized as a receiver (i.e., to receive an ultrasonic signal).Utilization of such a pitch-catch scheme may advantageously reduce, oreven eliminate, transducer ringing effects, by substantiallyelectromechanically isolating the transmitter and receiver, and therebymay significantly improve the signal to noise ratio of the transducer.One example of a transducer configured for pitch-catch ultrasonicmeasurements is shown in FIG. 6. Transducer 440 includes an innerpiezoelectric disk 442 and an outer piezoelectric ring 444 separated bya non piezoelectric (e.g., polymer) ring 446. In the embodiment shown,piezoelectric disk 442 may be utilized as a transmitter and electricallycoupled to suitable transmitter electronics, for example, via gold layer480A, while piezoelectric ring 444 may be utilized as a receiver andcoupled to suitable receiver electronics, for example, via gold layer480B. The artisan of ordinary skill will readily recognize thatpiezoelectric disk 442 may alternatively be utilized as a receiver andpiezoelectric ring 444 utilized as a transmitter. As withpiezo-composite transducers 240 and 340, (FIGS. 4 and 5) substantiallyany suitable piezoelectric and non piezoelectric materials may beutilized in fabricating transducer 440. In certain advantageousembodiments, the transmitter may be fabricated from a lead zirconatetitanate such as PZT5A available from Morgan Electro Ceramics while thereceiver may be fabricated from a lead metaniobate such as K81 or K85,both of which are available from Keramos Advanced Piezoelectrics.

It will be appreciated that substantially any piezo-composite structuremay be configured for such pitch-catch ultrasonic measurements, providedthat a transmitter portion of the transducer may be substantiallyelectromechanically isolated from a receiver portion thereof. Forexample, transducer 340, shown in FIG. 5, may be modified such thatpiezoelectric disk 342 and piezoelectric ring 344A are utilized as atransmitter and piezoelectric rings 344B, 344C, and 344D are utilized asa receiver. This may be accomplished, for example, by attaching separateleads to the transmitter and receiver portions of the piezo-composite,e.g., a first lead coupled to the piezoelectric disk 342 and ring 344Aand a second lead coupled to the piezoelectric rings 344B, 344C, and344D. Likewise, transducer 240, shown in FIG. 4, may be similarlymodified such that a portion of the piezoelectric posts 234 are utilizedas a transmitter (e.g., the inner posts) and another portion as areceiver (e.g., the outer posts). Of course, in such alternativeembodiments of FIGS. 4 and 5, gold layer 280 would have to be modifiedto provide separate, electromechanically isolated connections to thetransmitter and receiver portions.

Referring now to FIG. 7, and with further reference to FIG. 3, acousticsensor 120 is shown in further detail, including corresponding parts112, 122 and 124 from FIG. 3. Acoustic sensor 120 in this embodiment isa multi-layer device including a piezo-composite transducer 140. Asdescribed above, piezo-composite transducer 140 may includesubstantially any suitable piezo-composite such as one of the exemplaryembodiments described above with respect to FIGS. 4 through 6. As shownon FIG. 7, various embodiments of acoustic sensor 120 may optionallyinclude a backing layer 160 for substantially attenuating ultrasonicenergy reflected back into the transducer from other components insensor 120 (rather than outward into the drilling fluid). Variousembodiments of acoustic sensor 120 may optionally include a matchinglayer assembly 150 including at least one each of matching layers 152and 154 for providing impedance matching between the piezo-compositetransducer 140 and the drilling fluid at the exterior of the tool.Embodiments of the matching layer assembly 150 may also include abarrier layer 156 for shielding the piezo-composite transducer 140 frommechanical damage as described in more detail below.

With continued reference to FIG. 7, backing layer 160 typically includesa composite material having a mixture of one or more elastomeric polymermaterials (e.g., rubber) and one or more powder materials. Backing layer160 may include substantially any elastomeric polymer material,advantageously with sufficient high temperature resistance for use indownhole applications. Suitable elastomeric polymer materials alsoadvantageously provide sufficient dampening of back reflected ultrasonicenergy at downhole temperatures. Natural rubbers, for example, typicallyprovide sufficient dampening of ultrasonic energy at low temperatures.Various vulcanized rubbers (e.g., sulfur crosslinked elastomers)typically provide sufficient dampening of ultrasonic energy at highertemperatures and thus may be preferable in exemplary embodiments ofbacking layer 160.

Exemplary backing layers 160 may utilize fluoroelastomer polymers, whichgenerally provide exceptional resistance to high temperature aging anddegradation and thus tend to be well suited for meeting the demands ofthe downhole environment. Fluoroelastomers also tend to dampenultrasonic energy at temperatures up to and exceeding 250 degrees C.Fluoroelastomers are generally classified into four groups: A, B, F, andspecialty. The A, B, and F groups are known to generally have increasingfluid resistance derived from increased fluorine levels (about 66 atomicpercent, about 68 atomic percent, and about 70 atomic percent,respectively). Substantially any suitable A, B, F, and/or specialtyfluoroelastomer may be utilized in various embodiments of backing layer160. For example, exemplary backing layers 160 may include group Afluoroelastomers (i.e., those including about 66 atomic percentfluorine), such as Fluorel® brand fluoroelastomers FC 2178, FC 2181, FE5623Q, or mixtures thereof, available from Dyneon®, Decator, Ala. Otherexemplary backing layers may include copolymers of vinylidene fluorideand hexafluoropropylene, such as Viton® B-50, available from DuPont(® deNemours, Wilmington, Del.

Exemplary backing layers may also include substantially any suitablepowder material, such as tungsten powers, tantalum powders, and/orvarious ceramic powders. In one useful embodiment, tungsten powdershaving a bimodal particle size distribution may be utilized. Forexample, one exemplary backing layer includes a mixture of C-8 and C-60tungsten powders available from Alldyne Powder Technologies, 148 LittleCove Road, Gurley, Ala. The particle size of C8 is in the range fromabout 2 to about 4 microns while the particle size of C60 is in therange from about 10 to about 18 microns.

With further reference to FIG. 7, exemplary backing layers 160 mayfurther include one or more additives that may improve one or moreproperties of the backing layer 160. For example, acid acceptors arecommonly used in fluoroelastomer compounds and are known to enhance thehigh temperature performance of the fluoroelastomer. Commonly used acidacceptors include magnesium oxide (MgO), calcium hydroxide (CaOH2),litharge (PbO), zinc oxide (ZnO), dyphos (PbHPO3), and calcium oxide(CaO). Calcium oxide is also known to minimize fissuring, improveadhesion, and reduce mold shrinkage of fluoroelastomer compounds. Avariety of fillers may also be used, for example, to provide increasedviscosity, hardness, and strength. Common fillers for fluoroelastomersinclude various carbon blacks, such as MT Black N-990, available fromEngineered Carbons, Inc., P.O. Box 2831, Borger, Tex. Mineral fillers,such as barium sulfate, calcium silicate, titanium dioxide, calciumcarbonate, diatomaceous silica, and iron oxide may also be utilized.

Exemplary backing layers according to this invention have beenfabricated according to the following procedure: A bimodal mixture oftungsten powder was prepared by mixing about 1000 grams of C-8 tungstenpowder with about 2900 grams of C-60 tungsten powder, both of which areavailable from Alldyne Powder Technologies. The tungsten powder mixturewas cleaned by submerging in a solvent, such as acetone, draining thesolvent, and baking at about 160 degrees C. for two or more hours. Afluoroelastomer blend was then prepared by mixing about 300 grams ofFC-2181 with about 200 grams of FC-2178, both of which are availablefrom Dyneon®. About 15 grams of magnesium oxide, maglite powderavailable from Northwest Scientific Supply, Cedar Hill Road, Victoria,BC, Canada, about 70 grams of calcium oxide, R1414, available fromMalinckrodt Baker, 222 Red School Lane, Phillipsburg, N.J., about 15grams of a first carbon black, MT black N-990, and about 15 grams of asecond carbon black, N-774, both of which are available from EngineeredCarbons, and about 80 grams of a mold release, such as VPA2, availablefrom DuPont(® de Nemours, Wilmington, Del., were then added to andblended with the fluoroelastomer blend.

The fluoroelastomer blend, including the above additives, was dissolvedin about 1500 grams of a methyl isobutyl ketone (MIBK) solvent. Thetungsten powder mixture was then stirred into the solvent mixture. Themixture was stirred frequently (or continuously) to prevent settling ofthe tungsten powders until about 80 percent or more of the MIBK solventhad evaporated (typically about 1 to 2 hours). Stirring was thendiscontinued and the mixture allowed to sit for about 12 hours (e.g.,overnight) until substantially all of the remaining solvent had beenevaporated. The prepared material was then placed in a single cavitymold and hot pressed into the form of a pellet having a thickness ofabout 2.2 centimeters under a load of about 125,000 kilograms at atemperature of about 165 degrees C.

Backing layers fabricated as described above were found to haveexcellent stability under typically downhole conditions (e.g.,temperatures up to about 200 degrees C. and pressures up to about 25,000psi). Such backing layers were also found to provide greater than 50 dBattenuation of ultrasonic energy at a frequency band of about 100 kHz.

With further reference to FIG. 7, matching layer assembly 150 typicallyincludes at least one impedance matching layer 152 and a barrier layer156. In the embodiment of the matching layer assembly shown in acousticsensor 120, the matching layer assembly includes first and secondimpedance matching layers 152, 154. First impedance matching layer 152is typically disposed adjacent the piezo-composite transducer 140 andmay be characterized as having an acoustic impedance similar thereto,for example in the range of from about 8 to about 15 MRayl. In oneembodiment, first impedance matching layer 152 is fabricated from aglass ceramic, such as a Macor® glass ceramic available from CorningGlass Works Corporation, Houghton Park, N.Y. Glass ceramics mayadvantageously provide exceptional high temperature resistance as wellas a low coefficient of thermal expansion. Glass ceramics also tend topossess favorable mechanical properties and may also function to protectthe transducer assembly. In alternative embodiments, first impedancematching layer may be fabricated from a polymeric material (e.g., aconventional epoxy having a suitable acoustic impedance and hightemperature resistance). Such an epoxy may also advantageously includefillers, such as various ceramic particles, for reducing the thermalcoefficient of expansion and increasing the acoustic impedance of thelayer.

With continued reference to FIG. 7, second impedance matching layer 154is typically disposed adjacent the first impedance matching layer 152and may be characterized as having an acoustic impedance similar to thatof conventional drilling fluid, e.g., on the order of from about 3 toabout 7 MRayl. Embodiments of the second impedance matching layer mayalso be fabricated from conventional epoxy materials, such as Insulcast®125 available from Insulcast®. Alternative embodiments may be fabricatedfrom composite materials including a mixture of an epoxy and a glassceramic. For example, in one particular embodiment, a compositeincluding from about 40 to about 80 volume percent Insulcast® 125 andfrom about 20 to about 60 volume percent Macor® glass ceramic may beutilized. Such a composite may be fabricated, for example, by removingsections of a Macor® glass ceramic disk (e.g., by cutting grooves ordrilling holes) and by filling the openings with Insulcast® 125.

With continued reference to FIG. 7, matching layers 152 and 154 may besubstantially any thickness depending on the pulse frequency content ofthe transmitted ultrasonic energy. For typical downhole applications inwhich the frequency band of the transmitted ultrasonic energy is in therange of from about 100 to about 700 kHz, the thickness of the firstimpedance matching layer 152 is typically in the range from about 1 toabout 2 millimeters, while the thickness of the second impedancematching layer 154 is typically in the range from about 0.8 to about 1.5millimeters.

Referring now to FIG. 8A, it will be appreciated that the first andsecond impedance matching layers may be fabricated as an integral unit250. For example, in the embodiments shown, first and second impedancematching layers 152′ and 154′ may be fabricated from a single a glassceramic disk 252, e.g., a Macor® disk available from Corning GlassWorks. An array of holes 254 (or grooves, cuts, dimples, indentations,etc.) is formed in one face 255 of the disk 252 (for example, by adrilling or cutting operation). The other face 253 of the disk 252 wouldnot undergo such treatment. The holes 254 (or grooves) may penetrate tosubstantially any depth 257 into the disk, but typically penetrate fromabout 30 to about 60 percent of the depth thereof. The holes 254 (orgrooves, etc.) may further be filled, for example, with a polymer epoxy258, such as Insulcast® 125, effectively resulting in a two-layerstructure, a first impedance matching layer 152′ having a relativelyhigher acoustic impedance (e.g., from about 8 to 15 MRayl) and a secondimpedance matching layer 154′ having a relatively lower acousticimpedance (e.g., from about 3 to about 7 MRayl).

Referring now to FIG. 8B, an alternative embodiment of impedancematching layers is shown. FIG. 8B illustrates a single matching layer350 having an acoustic impedance that ranges from a relatively highervalue (e.g., from about 8 to about 15 MRayl) at a first face 353 torelatively lower value (e.g., from about 3 to about 7 MRayl) at a secondface 355. For example, in the embodiments shown, a series of grooves 354(or holes, cuts, dimples, indentations, etc.) may be formed in one face355 of a glass ceramic disk 352, such as a Macor® disk. As describedabove with respect to FIG. 8A, the grooves 354 (or holes, etc.) may befilled with a polymer epoxy 358 such as Insulcast® 125. The grooves 354are tapered such that the ratio of epoxy (groove or hole area) toceramic disk increases from the lower face 353 to the upper face 355thereof. As a result the acoustic impedance also tends to increase fromthe lower face 353 to the upper face 355, i.e., from about that of theceramic disk to a fraction thereof depending upon the area fraction ofthe grooves and the type of polymer epoxy utilized. The grooves 354 maypenetrate to substantially any depth 357 into the disk, but typicallypenetrate from about 60 to about 90 percent of the depth thereof.

During a typical logging while drilling (LWD) measurement cycle,downhole tools (in particular the acoustic sensors 120 disposed inmeasurement tool 100—FIGS. 1 through 3) may repeatedly impact thesidewall of the borehole or rock cuttings in the drilling fluid. Suchimpacts to the front face of an acoustic sensor are known in the art topotentially cause various data anomalies. In extreme cases, such impactsare further known to damage the sensors. Provision of a barrier layerhaving sufficient mechanical strength and wear resistance to minimizesuch damage may thus advantageously prolong the life of acoustic sensorsutilized in downhole environments and/or improve the reliability ofacoustic data generated thereby. Provision of such a barrier layer mayalso enable an outer surface of an acoustic sensor to be flush with anouter surface of the tool body (e.g., tool body 110 in FIG. 3), ratherthan recessed as in most prior art tools. Sensors provided flush ratherthan recessed may be advantageous for some downhole applications.

With further reference to FIG. 7, suitable barrier layers 156 may befabricated from substantially any material having sufficient strengthand wear resistance to adequately protect the piezo-composite transducer140. For example, metallic materials such as titanium and stainlesssteels may be utilized in embodiments of the barrier layer 156.Alternatively, fiber reinforced composites, such as fiberglass treatedwith an elastomeric coating, for example, may provide sufficientstrength to be utilized in various embodiments of the barrier layer 156.Desirable barrier layers 156 also typically possess sufficiently lowacoustic impedance, e.g., less than about 10 MRayl, so as not to overlyobstruct transmitted or received ultrasonic energy.

Referring now to FIG. 9A, a schematic representation of one embodimentof a barrier layer 260 is illustrated. Barrier layer 260 may befabricated, for example, from a titanium disk 262, although variousother materials such as stainless steels may also be suitable, having athickness, for example, in a range of from about 0.3 to about 1.2millimeters. Titanium, while having sufficient mechanical strength, alsoadvantageously includes a relatively low acoustic impedance (ascompared, for example, to ferrous materials such as various plain carbonsteels and stainless steels). Segmenting the barrier layer, for exampleas shown, may further reduce the acoustic impedance (e.g., to less than50 percent of that of a solid disk). In one desirable embodiment, atitanium disk 262 includes a plurality of concentric grooves 264 (orcuts, holes, etc.) formed in one face 266 thereof, with the grooves 264typically occupying from about 20 to about 40 percent of the crosssectional area of the disk 262. The grooves 294 are typically filled,for example, with a polymeric epoxy resin material 268, such asInsulcast® 125, available from Insulcast® or Viton®, available from E.I.Du Pont de Nemours Company, Wilmington, Del. It will be appreciated thatalternative groove patterns may also be utilized, such as, for example,two sets of orthogonal grooves. Embodiments of barrier layer 260 may be,for example, deployed as item 156 and bonded to the second impedancematching layer 154 (FIG. 7) using an adhesive such as Insulbond® 839,available from Insulcast®, with face 262 adjacent matching layer 154.

Referring now to FIG. 9B, a schematic representation of one alternativeembodiment of a barrier layer 360 is illustrated. Barrier layer 360 issimilar to barrier layer 260 (FIG. 8A) in that it is fabricated from atitanium disk (or alternatively a stainless steel or other metallicmaterial). Barrier layer 360, differs from that of barrier layer 260,however, in that it is corrugated, for example, by a stamping process.Barrier layer 360 includes a plurality, e.g., from about two to abouteight, concentric corrugated grooves 362 disposed therein. Thecorrugated grooves 362 tend to reduce the strength of the disk along itscylindrical axis 365 and thereby correspondingly tend to reduce theacoustic impedance of the barrier layer 360 (e.g., to less than 50percent of that of a solid disk). Barrier layer 360 may typically befabricated by a conventional stamping process (e.g., by stamping face364) and thus may also advantageously reduce fabrication costs. Barrierlayer 360 may also be deployed as item 156 and bonded to the secondimpedance matching layer 154 (FIG. 7), for example, using an adhesivesuch as Insulbond(® 839, available from Insulcast(®, with face 364adjacent matching layer 154.

Embodiments of the acoustic sensors of this invention may be fabricatedby substantially any suitable method. For example, exemplary embodimentsof acoustic sensor 120 (FIGS. 3 and 7) have been fabricated according tothe following procedure. A backing layer was prepared according to theprocedure described above. A 1-3 piezo-composite transducer was preparedaccording to the dice and fill procedure described above. Teflon(®coated leads were then attached to the faces of the transducer (e.g.,gold layers 280 in FIG. 4). The piezo-composite transducer was bonded toa front surface of the backing layer using a thin layer (about 0.1millimeter) of Insulbond® 839 adhesive, available from Insulcast. Amatching layer element was fabricated as described above with respect toFIG. 8A. One face (e.g., face 253 in FIG. 8A) of the matching layerelement was bonded to the upper surface of the piezo-compositetransducer using Insulbond® 839. A corrugated titanium barrier layer wasstamped as described above and bonded to the upper surface of thematching layer element using Insulbond(® 839. The Teflon® coated leadswere then inserted into a slot in the periphery of the backing layer andsoldered to corresponding pins mounted on the back side of the backinglayer. The sensor assembly was then inserted into a housing. An annularregion (e.g., annular region 125 in FIG. 7) around the sensor componentsand the housing was then filled (e.g., via conventional vacuum filling)with Insulcast® 125 epoxy. A molded Viton® bond seal (e.g., seal 114 inFIG. 7) was then applied around the outer periphery of the annularregion.

Referring now to FIG. 10, a schematic representation of an alternativeembodiment of an acoustic sensor 120′ is illustrated. Acoustic sensor120′ is substantially similar to that of acoustic sensor 120 (FIGS. 3and 7) in that it includes a piezo-composite transducer element 140 andother correspondingly-numbered parts. Acoustic sensor 120′ differs fromacoustic sensor 120 (FIG. 7) in that annular region 125′ includes apressure equalization layer 170 disposed inside the housing 122 andaround the sensor components (e.g., components 140, 152, 154, 160, and162). The pressure equalization layer 170 may include, for example, athin (e.g. about 0.3 millimeter) layer of silicone oil and mayadvantageously function to substantially evenly distribute boreholepressure changes about the sensor components. Sensor 120′ furtherdiffers from sensor 120 (FIG. 7) in that it includes a second backinglayer 162 fabricated from a material having a negative thermal expansioncoefficient, such as NEX-I or NEX-C glass ceramic available from OharaCorporation, 23141 Arroyo Vista, Santa Margarita, Calif. Negativethermal coefficient backing layers may advantageously reduce internalstresses resulting from borehole temperature fluctuations and mayprovide further attenuation of back reflected acoustic energy. Sensor120′ still further differs from sensor 120 (FIG. 7) in that an outerdiameter of the barrier layer 156′ is chosen to be substantially flushwith an outer diameter of the housing. Barrier layer 156′ is furthertypically welded 116 to housing 122 and effectively functions as afaceplate.

While FIGS. 3, 7, and 10 depict acoustic sensors includingpiezo-composite transducer elements, it will be appreciated that variousembodiments of this invention may include a conventional piezo-ceramictransducer element rather than a piezo-composite transducer element. Forexample, backing layer 160 may advantageously (as compared to prior artbacking layers) be utilized in acoustic sensors having conventionalpiezo-ceramic transducer elements. Likewise, matching layer assembly 150may advantageously (as compared to prior art matching layers) beutilized in acoustic sensors having conventional piezo-ceramictransducer elements.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalternations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A downhole measurement tool, comprising: a substantially cylindrical tool body having a cylindrical axis; at least one acoustic sensor deployed on the tool body, the acoustic sensor including a piezo-composite transducer element with anterior and posterior faces, the piezo-composite transducer in electrical communication with an electronic control module via conductive electrodes disposed on each of said faces; and the piezo-composite transducer element including regions of piezoelectric material deployed in a matrix of a substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension.
 2. The downhole tool of claim 1, wherein the piezoelectric material is selected from the group consisting of lead zirconate titanates and lead metaniobates.
 3. The downhole tool of claim 1, wherein the piezoelectric material has a Curie temperature greater than or equal to about 250 degrees C.
 4. The downhole tool of claim 1, wherein the piezoelectric material has a coupling coefficient of greater than or equal to about 0.3.
 5. The downhole tool of claim 1, wherein the non piezoelectric material is a polymeric material.
 6. The downhole tool of claim 5, wherein the polymeric material is an epoxy resin.
 7. The downhole tool of claim 5, wherein the polymeric material has a coefficient of thermal expansion less than about 100 parts per million per degree C.
 8. The downhole tool of claim 5, wherein the polymeric material has a glass transition temperature of greater than about 250 degrees C.
 9. The downhole tool of claim 1, wherein the regions of piezoelectric material comprise a periodic array of spaced piezoelectric material posts.
 10. The downhole tool of claim 9, wherein the piezo-composite transducer element is a product of the process comprising: providing a piezo-ceramic disk having first and second faces; cutting a first set and a second set of grooves in the first face, the grooves in the first set being substantially orthogonal to the grooves in the second set, wherein removal of piezo-ceramic material in said groove cutting is operative to shape the piezoelectric material posts; casting the non piezoelectric material into the grooves to form, in combination with the piezo-ceramic disc, a specimen of piezo-composite material having first and second faces corresponding substantially to those of the piezo-ceramic disk; polishing the specimen to a predetermined thickness; and disposing conductive electrodes on each of the first and second faces of the specimen.
 11. The downhole tool of claim 1, wherein the piezo-composite transducer element comprises a laminate including alternating layers of the piezoelectric material and the non piezoelectric material.
 12. The downhole tool of claim 1, wherein the piezo-composite transducer element includes alternating concentric rings of the piezoelectric material and the non piezoelectric material disposed about a central element of the piezoelectric material.
 13. The downhole tool of claim 12, comprising from about 2 to about 8 rings of the piezoelectric material.
 14. The downhole tool of claim 12, wherein each of the rings of the piezoelectric material has a predetermined radial thickness selected to decrease with increasing distance from the central element.
 15. The downhole tool of claim 14, wherein the radial thickness of the rings of piezoelectric material is selected to decrease according a mathematical function selected from the group consisting of Bessel functions and Gaussian functions.
 16. The downhole tool of claim 12, wherein the piezo-composite transducer element comprises a periphery and a plurality of axial slots disposed around the periphery.
 17. The downhole tool of claim 12, wherein the piezo-composite transducer element comprises a periphery and first, second, third, and fourth axial slots disposed substantially equidistantly around the periphery.
 18. The downhole tool of claim 12, wherein the piezo-composite transducer element is a product of the process comprising: providing a piezo-ceramic slurry; casting the piezo-ceramic slurry in a reverse mold to form the concentric rings of piezoelectric material about the central element; casting the non piezoelectric material in open spaces between the concentric rings of piezoelectric material to form, in combination with the concentric rings of piezoelectric material, a specimen of piezo-composite material having first and second faces; polishing the specimen to a predetermined thickness; and disposing conductive electrodes on each of the first and second faces of the specimen.
 19. The downhole tool of claim 1, wherein the piezo-composite transducer element includes first and second separate piezoelectric elements, the first piezoelectric element serving as a transmitter, the second piezoelectric element serving as a receiver.
 20. The downhole tool of claim 19, wherein the first and second piezoelectric elements are substantially isolated electromechanically from one another by a non piezoelectric material.
 21. The downhole tool of claim 19, wherein the first piezoelectric element includes a lead zirconate titanate piezoelectric material and the second piezoelectric element includes a lead metaniobate piezoelectric material.
 22. The downhole tool of claim 1, wherein the conductive electrodes comprise gold.
 23. The downhole tool of claim 1, in which the piezo-composite transducer element is deployed in a housing, and further comprising a pressure equalization layer disposed on an interior surface of the housing.
 24. The downhole tool of claim 23, wherein the pressure equalization layer includes silicone oil.
 25. The downhole tool of claim 1, in which the at least one acoustic sensor comprises first, second, and third acoustic sensors, each acoustic sensor including corresponding first, second, and third piezo-composite transducer elements.
 26. The downhole tool of claim 25, in which the tool body has a periphery, and wherein the first, second, and third acoustic sensors are disposed substantially equidistantly about the periphery of the tool body.
 27. The downhole tool of claim 1, wherein the electronic control module is deployed in the tool body.
 28. The downhole tool of claim 1, in which the tool body is couplable with a drill string.
 29. The downhole tool of claim 1, in which the downhole tool is selected from the group consisting of a logging while drilling tool and a measurement while drilling tool.
 30. The downhole tool of claim 1, wherein the at least one acoustic sensor further comprises a laminate including a backing layer deployed nearer to the cylindrical axis from the piezo-composite transducer.
 31. The downhole tool of claim 1, wherein the at least one acoustic sensor further comprises a laminate including at least one matching layer deployed further away from the cylindrical axis than the piezo-composite transducer.
 32. The downhole tool of claim 1, wherein the at least one acoustic sensor further comprises a laminate including the piezo-composite transducer and a barrier layer, the barrier layer deployed on an outermost surface of the laminate furthest away from the cylindrical axis.
 33. An acoustic sensor, comprising: a laminate including a piezo-composite transducer element having first and second faces, the laminate further including a composite backing layer deployed on the first face of the piezo-composite transducer element; the piezo-composite transducer element including regions of piezoelectric material deployed in a matrix of a substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension; the piezo-composite transducer element including conductive electrodes disposed on the first and second faces thereof; and the composite backing layer comprising at least one powder material disposed in an elastomeric matrix material.
 34. The acoustic sensor of claim 33, wherein the at least one powder material comprises at least one tungsten powder.
 35. The acoustic sensor of claim 34, wherein the at least one powder material comprises first and second tungsten powders, the first tungsten powder having an average particle size greater than that of the second tungsten powder.
 36. The acoustic sensor of claim 35, wherein: the first tungsten powder has average particle size ranging from about 2 to about 4 microns; and the second tungsten powder has an average particle size ranging from about 10 to about 18 microns.
 37. The acoustic sensor of claim 33, wherein the elastomeric material comprises a fluoroelastomer material.
 38. The acoustic sensor of claim 37, wherein the fluoroelastomer material comprises about 66 atomic percent fluorine.
 39. The acoustic sensor of claim 37, wherein the fluoroelastomer material comprises about 68 atomic percent fluorine.
 40. The acoustic sensor of claim 37, wherein the fluoroelastomer material comprises about 70 atomic percent fluorine.
 41. The acoustic sensor of claim 37, wherein the fluoroelastomer material includes a copolymer of vinylidene fluoride and hexafluoropropylene.
 42. The acoustic sensor of claim 37, wherein the composite backing layer further comprises at least one acid accepter selected from the group consisting of magnesium oxide, calcium hydroxide, litharge, zinc oxide, dyphos, and calcium oxide.
 43. The acoustic sensor of claim 37, wherein the composite backing layer further comprises at least one carbon black filler.
 44. The acoustic sensor of claim 37, wherein the composite backing layer further comprises at least one mineral filler selected from the group consisting of barium sulfate, calcium silicate, titanium dioxide, calcium carbonate, diatomaceous silica, and iron oxide.
 45. The acoustic sensor of claim 37, wherein the composite backing layer is a product of the process comprising: dissolving the fluoroelastomer material in a liquid solvent; mixing one or more tungsten powders into the solvent; substantially evaporating the solvent to form a specimen of fluoroelastomer composite material; and forming the composite backing layer by hot pressing the specimen into a pellet shape.
 46. The acoustic sensor of claim 33, further comprising an additional backing layer disposed adjacent the composite backing layer, the additional backing layer having a negative coefficient of thermal expansion.
 47. The acoustic sensor of claim 46, wherein the additional backing layer comprises a glass ceramic material.
 48. The acoustic sensor of claim 46, wherein the composite backing layer is interposed between the piezo-composite transducer element and the additional backing layer.
 49. The acoustic sensor of claim 33, wherein: the regions of piezoelectric material include a periodic array of spaced piezoelectric material posts; the piezoelectric material is selected from the group consisting of lead zirconate titanates and lead metaniobates and has a Curie temperature greater than or equal to about 250 degrees C.; and the non piezoelectric material includes an epoxy resin.
 50. The acoustic sensor of claim 33, wherein the laminate further comprises one or more matching layers deployed proximate the second face of the piezo-composite transducer element.
 51. An acoustic sensor, comprising: a laminate including a piezo-composite transducer element having first and second faces, the laminate further including a matching layer assembly deployed on the second face of the piezo-composite transducer element; the piezo-composite transducer element including regions of piezoelectric material deployed in a matrix of a substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension; the piezo-composite transducer element including conductive electrodes disposed on the first and second faces thereof; and the matching layer assembly including at least one matching layer and at least one barrier layer, the barrier layer being formed from a metallic material, the at least one matching layer being deployed between the piezo-composite transducer element and the barrier layer.
 52. The acoustic sensor of claim 51, wherein the at least one matching layer comprises first and second matching layers, the first matching layer being deployed between the piezo-composite transducer element and the second matching layer.
 53. The acoustic sensor of claim 52, wherein the first matching layer has an acoustic impedance in the range from about 8 to about 14 MRayl.
 54. The acoustic sensor of claim 52, wherein the first matching layer comprises an epoxy resin.
 55. The acoustic sensor of claim 54, wherein the first matching layer comprises a composite mixture of the epoxy resin and a ceramic material.
 56. The acoustic sensor of claim 52, wherein the first matching layer comprises a ceramic material.
 57. The acoustic sensor of claim 52, wherein the second matching layer has an acoustic impedance in the range from about 3 to about 7 MRayl.
 58. The acoustic sensor of claim 52, wherein the second matching layer comprises an epoxy resin.
 59. The acoustic sensor of claim 52, wherein the second matching layer comprises a composite mixture of an epoxy resin and a ceramic material.
 60. The acoustic sensor of claim 52, wherein the first matching layer and the second matching layer are formed from a single glass ceramic work piece.
 61. The acoustic sensor of claim 60, wherein the glass ceramic work piece has a plurality of openings formed in one face thereof, the openings being filled with an epoxy resin.
 62. The acoustic sensor of claim 61, wherein the openings are selected from the group consisting of holes, cuts, grooves, dimples, and indentations.
 63. The acoustic sensor of claim 61, wherein the plurality of openings comprise from about 40 to about 80 volume percent of the second matching layer.
 64. The acoustic sensor of claim 51, wherein the at least one matching layer comprises a single matching layer having an acoustic impedance that decreases from a relatively higher value at a first face of the matching layer to a relatively lower value at a second face of the matching layer.
 65. The acoustic sensor of claim 64, wherein the single matching layer comprises a glass ceramic disk having a plurality of openings formed in one face of the matching layer, the openings being filled with an epoxy resin.
 66. The acoustic sensor of claim 65, wherein the openings are tapered such that an area ratio of the epoxy resin to the glass ceramic increases from the first face to the second face.
 67. The acoustic sensor of claim 51, wherein the metallic material is selected from the group consisting of stainless steel and titanium.
 68. The acoustic sensor of claim 51, wherein the metallic material comprises titanium.
 69. The acoustic sensor of claim 51, wherein the barrier layer has an acoustic impedance less than about 10 MRayl.
 70. The acoustic sensor of claim 51, wherein the barrier layer is corrugated.
 71. The acoustic sensor of claim 70, wherein said corrugated barrier layer is formed by a metal stamping process.
 72. The acoustic sensor of claim 51, wherein the barrier layer comprises a composite material including a metallic work piece including opposing faces, the work piece having a plurality of openings formed in one of the faces thereof, the plurality of openings being filled with an epoxy resin.
 73. The acoustic sensor of claim 72, wherein the openings are selected from the group consisting of holes, cuts, and grooves.
 74. The acoustic sensor of claim 72, wherein the openings comprise a plurality of concentric grooves.
 75. The acoustic sensor of claim 51, wherein the barrier layer is welded to a sensor housing.
 76. The acoustic sensor of claim 51, wherein: the regions of piezoelectric material include a periodic array of spaced piezoelectric material posts; the piezoelectric material is selected from the group consisting of lead zirconate titanates and lead metaniobates and has a Curie temperature greater than or equal to about 250 degrees C.; and the non piezoelectric material includes an epoxy resin.
 77. The acoustic sensor of claim 60, further comprising a backing layer deployed proximate the first face of the piezo-composite transducer element.
 78. An acoustic sensor, comprising: a laminate including a piezo-composite transducer element having first and second faces, the laminate further including a composite backing layer deployed on the first face of the piezo-composite transducer element and a matching layer assembly deployed on the second face of the piezo-composite transducer assembly; a piezo-composite transducer element including regions of piezoelectric material disposed in a matrix of a substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension; the piezo-composite transducer element including conductive electrodes disposed on the first and second faces thereof; the composite backing layer including at least one powder material disposed in a fluoroelastomer matrix material; and the matching layer assembly including at least one matching layer and a barrier layer, the barrier layer including a metallic material, the at least one matching layer being deployed between the piezo-composite transducer element and the barrier layer.
 79. The acoustic sensor of claim 78, wherein: the regions of piezoelectric material include a periodic array of spaced piezoelectric posts; the piezoelectric material is selected from the group consisting of lead zirconate titanates and lead metaniobates and has a Curie temperature greater than or equal to about 250 degrees C.; and the non piezoelectric material includes an epoxy resin.
 80. The acoustic sensor of claim 78, wherein: the powder material includes a tungsten powder; the matching layer assembly includes first and second matching layers, the first matching layer being deployed between the piezo-composite transducer element and the second matching layer, the first matching layering having an acoustic impedance in the range from about 8 to about 15 MRayl, and the second matching layer having an acoustic impedance in the range from about 3 to about 7 MRayl; and the barrier layer includes corrugated titanium.
 81. An acoustic sensor, comprising: a piezo-composite transducer element including regions of piezoelectric material deployed in a matrix of a substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension; the piezoelectric material having a Curie temperature greater than or equal to about 250 degrees C.; the piezo-composite transducer element including conductive electrodes disposed on first and second faces thereof; and the acoustic sensor being configured for use in a downhole measurement tool.
 82. A method for fabricating a downhole measurement tool, the method comprising: (a) providing a substantially cylindrical tool body having an electronic control module, the tool body being couplable with a drill string; (b) providing at least one acoustic sensor including a piezo-composite transducer element with anterior and posterior faces, the piezo-composite transducer element including regions of piezoelectric material deployed in a matrix of substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension; the piezo-composite transducer element further including conductive electrodes disposed on each of said faces; (c) deploying the at least one acoustic sensor on the tool body in electrical communication with the electronic control module via said conductive electrodes, the at least one acoustic sensor operable to transmit and receive acoustic signals in a borehole.
 83. A method for fabricating an acoustic sensor, the method comprising: (a) forming a piezo-composite transducer element having first and second faces and including regions of piezoelectric material deployed in a matrix of substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension; (b) disposing conductive electrodes on the first and second faces of the piezo-composite transducer element; (c) forming a composite backing layer having at least one powder material disposed in an elastomeric matrix material; and (d) deploying the composite backing layer on the first face of the piezo-composite transducer element, the composite backing layer operable to substantially attenuate acoustic energy back reflected into the acoustic sensor.
 84. A method for fabricating an acoustic sensor, the method comprising: (a) forming a piezo-composite transducer element having first and second faces and including regions of piezoelectric material deployed in a matrix of substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension; (b) disposing conductive electrodes on the first and second faces of the piezo-composite transducer element; (c) forming at least at least one matching layer and a barrier layer, the barrier layer including a metallic material; (d) deploying the at least one matching layer on the second face of the piezo-composite transducer element; and (e) deploying the barrier layer proximate the at least one matching layer, the at least one matching layer being deployed between the piezo-composite transducer element and the at least one barrier layer.
 85. A method for fabricating an acoustic sensor, the method comprising: (a) forming a piezo-composite transducer element having first and second faces and including regions of piezoelectric material deployed in a matrix of substantially non piezoelectric material, the regions extending through a thickness of the transducer element in at least one dimension; (b) disposing conductive electrodes on the first and second faces of the piezo-composite transducer element; (c) forming a composite backing layer having at least one powder material disposed in an elastomeric matrix material; (d) forming at least at least one matching layer and a barrier layer, the barrier layer including a metallic material; (e) deploying the composite backing layer on the first face of the piezo-composite transducer element; (f) deploying the at least one matching layer on the second face of the piezo-composite transducer element; and (g) deploying the barrier layer proximate the at least one matching layer, the at least one matching layer being deployed between the piezo-composite transducer element and the at least one barrier layer. 